The present invention relates to a pressure sensor employing a semiconductor strain gauge, and more particularly to a pressure sensor including zero-point temperature compensation means.
Various gauges have been used for measuring masses, stresses, fluid pressures, etc. Among all, semiconductor strain gauges of high sensitivites utilizing the piezoresistance effects of semiconductors have been extensively applied in recent years.
The semiconductor strain gauge exploiting the piezoelectric effect of a semiconductor has the advantage that the rate of variation of a resistance to a strain, namely, the gauge factor is high. On the other hand, however, the gauge has the disadvantage that the resistance and the gauge factor thereof exhibit temperature dependencies of great magnitudes, resulting in unstable operations.
In general, the resistance R of a semiconductor strain gauge is given by the following equation: EQU R=R.sub.0 (1+.alpha.T){1+S.gamma.(1+.beta.T)} (1)
where R.sub.0 denotes the resistance of the unstrained gauge at a predetermined temperature, T the temperature of the semiconductor strain gauge, S the magnitude of a strain, .alpha. the temperature coefficient of the resistance of the gauge, .beta. the temperature coefficient of the gauge factor of the gauge, and .gamma. the gauge factor. The gauge factor .gamma. has its value and its sign determined depending upon the orientation of a semiconductor single crystal, an angle defined between a current and a stress within the gauge, etc.
Equation (1) is expanded as follows: EQU R=R.sub.0 (1+.alpha.T)+R.sub.0 (1+.alpha.T)(1+.beta.T)S.gamma.(2) EQU .perspectiveto.R.sub.0 (1+.alpha.T)+R.sub.0 {1+(.alpha.+.beta.)T}S.gamma.(3)
The second term of the right-hand side of Equation (2) indicates the variation of the gauge resistance based on the strain. The coefficient .alpha. varies depending upon an impurity density within the crystal of the semiconductor strain gauge and has, for example, a value of 3000-600 ppm/.degree.C. in case of the single crystal of silicon, while the coefficient .beta. does not depend upon the impurity density and has a value of about -2000 ppm/.degree.C. in the case of the silicon single crystal. The variation of the gauge resistance can have its temperature dependency diminished because, as apparent from the second term of Equation (3), the temperature coefficient .alpha. of the resistance of the semiconductor strain gauge and the temperature coefficient .beta. of the gauge factor thereof can be canceled by properly selecting the impurity density of the crystal.
A plurality of semiconductor strain gauges as described above constitute a strain-electric signal conversion bridge. The output of the bridge at the time of null (0) strain varies with a temperature change, or exhibits the so-called temperature dependency, on account of dispersions in the resistances R.sub.0 of the gauges and the temperature coefficients .alpha. thereof. This temperature dependency is the "zero-point temperature dependency", and it is the "zero-point temperature compensation" that reduces and compensates such temperature dependency.
As an expedient for the zero-point temperature compensation, the inventors have proposed a system disclosed in the specification of Japanese Patent Application No. 54-20847 (1979). This application was laid open as Japanese Patent Application Laying-open No. 55-113904 on Sept. 2, 1980. The corresponding U.S. application was filed as Ser. No. 121,093 on Feb. 13, 1980, and has been allowed as U.S. Pat. No. 4,337,665 under the date of July 6, 1982. The corresponding West-German application was filed as Application No. 3007142. 2-52 on Feb. 26, 1980, and is now pending.
With this system, however, a complicated circuit arrangement is involved. Accordingly, the system is problematic in point of reliability. More specifically, a circuit shown in FIG. 2 of U.S. Pat. No. 4,337,665 includes four amplifiers. Further, it includes fifteen resistors. A pressure sensor obtained by actually fabricating the circuit of FIG. 2 has included two transistors, twenty-five resistors, two thermistors and four capacitors, in addition to the four amplifiers.
Besides, the zero-point temperature compensation is performed by the use of an active circuit. More specifically, voltages at the middle points a and b of the two arms of semiconductor strain gauges are applied to the plus inputs of differential amplifiers 19 and 20, the minus inputs of which is supplied with a current for the zero-point temperature compensation. The zero-point temperature compensation is performed using the differential amplifiers 19 and 20, that is, using active elements. Also an amplifier 6 is used for effecting the temperature compensation with a constant current drive system.